(1) Field of the Invention
The present invention relates to noise light elimination technology for eliminating noise light components contained in a signal light, in the field of optical communications. In particular, the present invention relates to a method and apparatus for eliminating a noise light with the condition of the light remaining as is, using stimulated Brillouin scattering (SBS), and to an optical transmission system using such method and apparatus.
(2) Background Art
In recent wavelength division multiplexing (WDM) optical systems, long distance optical transmission systems, in general, referred to as long haul systems, in which for example the number of optical repeaters constructed using erbium doped fiber amplifiers (EDFAs) or the like is increased, or a repeating distance is extended by the introduction of Raman amplifiers, have made an appearance. Specifically, a maximum distance from the sending section to receiving section of signal light, is around 500 km to 1000 km in the conventional system. However, in the long haul system, this now exceeds 2000 km to 4000 km.
Incidentally, in the above-mentioned long haul system, an ASE (Amplified Spontaneous Emission) noise light due to a spontaneous emission light generated in the EDFA, or an ASS (Amplified Spontaneous Scattering) noise light due to Raman scattering of excitation light generated in the Raman amplifier is accumulated in each optical repeater. This accumulation of noise light degrades an optical SN ratio (OSNR), and also degrades signal light discrimination characteristics in an optical receiver, thus restricting extension of the repeating distance of the system.
Components of the abovementioned ASE or ASS noise light are generated at a width of several 10 nm in a wavelength band of a main signal light. For dealing with such noise light components, the existing system adopts a method for converting a light into electrical signals at each required repeating span and then generating a signal light that has been again modulated, to transmit, or a method for separating a WDM signal light into optical signals for each channel and then multiplexing the optical signals to transmit the multiplexed light, so as to eliminate the noise light components, thereby achieving a long distance transmission.
FIG. 18 is a block diagram showing a schematic construction of a conventional WDM optical transmission system that performs noise light elimination based on optic-electric conversion. In this conventional system, an optical transmission device 100 wavelength multiplexes, with a multiplexer (MUX) 102, optical signals of respective wavelengths output from a plurality of optical senders (OS1 to OSN) 101, to transmit the generated WDM signal light to an optical transmission path 103. The WDM signal light is then repeatedly transmitted towards an optical receiving device (not shown in the figure) while being amplified by a plurality of optical repeaters 104 arranged at required intervals on the optical transmission path 103. At this time, the accumulated noise light components generated in the respective optical repeaters are eliminated based on the optic-electric conversion by means of a noise light elimination device 300 arranged for each of the optical repeaters of “n” in number. Specifically, in the noise light elimination device 300, the WDM signal light in which is accumulated the noise light, output from the “n”th stage optical repeater 104, is demultiplexed into optical signals of respective wavelengths by a demultiplexer (DMUX) 301, to be converted into electrical signals in the optical receivers (OR) 302 corresponding to the respective wavelengths. Then, optical signals of respective wavelengths that have been modulated in accordance with the electrical signals output from the respective optical receivers 302 are output from optical senders (OS1 to OSN) 303, and wavelength multiplexed in a multiplexer (MUX) 304. As a result, the WDM signal light with the noise light components eliminated, is output to the optical transmission path 103.
Furthermore, FIG. 19 is a block diagram showing a schematic construction of a conventional WDM optical transmission system that performs noise light elimination based on the demultiplexing and multiplexing of WDM signal light. In this conventional system, the accumulated noise light components generated in the respective optical repeaters 104 are eliminated based on the demultiplexing and multiplexing of WDM signal light by means of a noise light elimination device 400 arranged for each of the optical repeaters 104 of “n” in number. Specifically, in the noise light elimination device 400, the WDM signal light in which is accumulated the noise light, output from the n-th stage optical repeater 104 is demultiplexed by a demultiplexer (DMUX) 401 having a narrow transmission band corresponding to the center wavelength of each channel, and the optical signals of each channel with the noise light components outside of the transmission eliminated, are multiplexed by a multiplexer (MUX) 402 to be output to the optical transmission path 103.
However, in these methods for noise light elimination in the conventional WDM optical transmission system as mentioned above, there are the following problems. At first, in the case of noise light elimination based on optic-electric conversion as shown in FIG. 18, there is a possibility that power variation (tilt) will occur in each of the wavelength lights of the WDM signal light output from the noise light elimination device 300, and hence a function for correcting this tilt must be furnished in the noise light elimination device 300. Specifically, for the noise light elimination device 300, a variable optical attenuator that adjusts the power for each signal light of each channel, or a spectrum analyzer unit (SAU) for monitoring the optical signal power of each wavelength, and electric circuits associated with these must be provided. Using this noise light elimination device 300 thus invites a cost increase for the overall system.
Furthermore, if the repeating span in which the noise light elimination device 300 is positioned becomes long, then at the time of optic-electric conversion, the noise light components contained in the input light of the optical receiver 302 become large so that the OSNR is degraded. Therefore, there is also a problem of the likelihood of errors occurring in the signal decision process in the optical receiver 302.
In the case of the noise light elimination based on the demultiplexing and multiplexing of the WDM signal light as shown in FIG. 19, then compared to the case for the noise light elimination based on the optic-electric conversion, it is no longer necessary to provide the optical receiver and the optical sender in the noise light elimination device 400. However, since the above-mentioned tilt correction for the signal light of each channel is similarly necessary, the device construction becomes complex as a result of providing a variable optical attenuator or SAU or the like, leading to a cost increase for the overall system.
Furthermore, by demultiplexing the WDM signal light for each channel, noise light components of wavelengths different to the signal wavelengths can be eliminated. However, noise light components of wavelengths same as the signal wavelength cannot be eliminated. Therefore, the noise light components of the same wavelength as the signal wavelength are transmitted unchanged together with the signal light from the noise light elimination device 400 to the optical transmission path 103. Hence, if the number of times of repeating operations is increased, the OSNR is degraded, so that a considerable degradation in transmission characteristics occurs.
Furthermore, in the noise light elimination device 400, the WDM signal light output from the optical repeater 104 of former stage passes through the demultiplexer 401 and a multiplexer 402 having in total an insertion loss of around 3 to 6 dB, or a variable optical attenuator for tilt correction having an insertion loss of around several dB, and is sent to the optical transmission path 103. Therefore, there is also a possibility that the optical signal power of each wavelength input to the latter stage optical repeater 104 becomes a low power that does not satisfy the input dynamic range of the optical amplifier to be used for the optical repeater 104.